Tris(μ2-2-methoxy-6-{[(2-sulfidoethyl)imino]methyl}phenolato)trinickel(II) dimethylformamide monosolvate: crystal structure, spectroscopic characterization and antibacterial activity

The crystal structure of a trinuclear NiII complex with a Schiff base ligand formed in situ from cysteamine (2-aminoethanethiol) and o-vanillin is reported as well results of its antibacterial activity screening.

The title trinuclear nickel(II) complex, [Ni 3 (C 10 H 11 NO 2 S) 3 ]ÁC 3 H 7 NO, with a Schiff base ligand formed in situ from 2-aminoethanethiol and o-vanillin crystallizes in the orthorhombic space group Pbca. Its asymmetric unit consists of one neutral Ni 3 L 3 molecule and one DMF solvent molecule. The solid-state organization of the complex can be described as an insertion of the solvent molecules within the crystallographically independent trinuclear Ni II species. Several C-HÁ Á Á edge-to-face interactions andstacking interactions link the components in the crystal. A first example of a short intermolecular C-HÁ Á ÁNi contact is found. Antibacterial in vitro screening revealed that the title compound has antibacterial activity, the best effect being against Acinetobacter baumannii.

Chemical context
Schiff base ligands are one of the most widely utilized classes of ligands in metal coordination chemistry because of their preparative accessibility, structural variety and strong metalbinding ability with many metal ions via azomethine HC N or phenolic groups (Garnovskii et al., 1993;Bera et al., 1998;Prabhakaran et al., 2004). o-Vanillin-based Schiff ligands demonstrate an exceptionally rich coordination chemistry and diverse properties -magnetism, luminescence, chirality, catalysis, cytotoxicity and ferroelectricity (Andruh, 2015). The N and S atoms play a key role in the coordination of metals at the active sites of numerous metallobiomolecules. It has been shown that ONS Schiff bases are moderately active against leukemia (Tofazzal et al., 2000). In particularly, nickel complexes with a multidentate NSO-containing mixed-ligand environment attract attention because such complexes play an important role in bioinorganic chemistry and redox enzyme systems and can be considered as model objects for studying the active sites of biological systems (Halcrow et al., 1994). In this work we present the crystal structure of a novel trinuclear Ni II complex with an NSO-type Schiff base ligand derived from o-vanillin and 2-aminoethanethiol as well results of its antibacterial activity screening against several Gram-positive and Gram-negative bacteria.

Structural commentary
The title complex crystallizes in the orthorhombic space group Pbca. The asymmetric unit consists of one neutral Ni 3 L 3 ISSN 2056-9890 molecule and one DMF solvent molecule. The molecular structure of the trinuclear complex unit is depicted in Fig. 1.
The coordination geometry around each Ni II ion can be described as slightly distorted square planar and is comparable to that found in similar complexes reported previously (Kaasjager et al., 2001;Constable et al., 2011). Each Ni II ion is tetracoordinated by an identical NOS 2 ligand environment: the dianionic Schiff base ligand occupies three of the four coordination sites (NOS), the fourth site place being filled by a bridging sulfur atom of a neighboring ligand. The deviation of the Ni II atom from the NOS 2 mean plane is 0.0927 (14) Å . Thus, the molecule has a 'crown' or bowl shape with the Ni 3 S 3 unit as its base in a distorted chair conformation. The torsion angles [between 78.49 (5) and 84.79 (5) ] deviate significantly from the ideal chair conformation for c-hexane which has torsion angles of 60 . For the Ni2 atom in this core, one additional short contact should be noted, C27-H27CÁ Á ÁNi( 1 2 + x, y, 1 2 À z) with an HÁ Á ÁNi distance of 2.58 Å .

Supramolecular features
The solid-state organization of the complex can be described as an insertion of the solvent molecules within the crystallographically independent trinuclear Ni II species (Fig. 2).
In this structure, in contrast to previously reported analogous complexes (Constable et al., 2011), the short C-HÁ Á ÁNi The molecular structure of the trinuclear complex unit of the title compound, showing 50% probability displacement ellipsoids.

Figure 2
The crystal packing of the title compound. H atoms are not shown for clarity. Table 1 Hydrogen-bond geometry (Å , ).

Database survey
A search of the Cambridge Structural Database (Version 5.38; last update November 2016; Groom et al., 2016) for related complexes with HÁ Á ÁNi contacts gave nine hits with intramolecular E-HÁ Á ÁNi contacts where E is mainly Ir, Rh, B and only two examples with C-HÁ Á ÁNi. There were no E-HÁ Á ÁNi intermolecular contacts found. A search for complexes with an Ni atom and an ONS Schiff base ligand gave 16 hits, including four closely related structures, viz. (Kaasjager et al., 2001;Constable et al., 2011).

Synthesis and crystallization
A solution of KOH (0.22 g, 4 mmol) in a minimum amount of methanol (2-3 ml) was added to a solution of 2-amino-ethanethiol hydrochloride (0.44g, 4 mmol) in methanol (5 ml) and stirred in an ice bath for 10 min. The white precipitate of solid KCl was removed by filtration and o-vaniline (0.61 g, 4 mmol) in ethanol (5 ml) was added to the filtrate and stirred on air magnetically for 2 h. Nickel acetate tetrahydrate (0.99 g, 4 mmol) in ethanol (6 ml) was added to the yellowish solution of the Schiff base formed in situ, and the resulting deep-brown solution was stirred magnetically and heated at 340-347 K for 1.5 h resulting in a dark-colored precipitate. The product was isolated by filtration, washed with dry i=i PrOH and finally dried in vacuo. Crystals suitable for crystallographic study were grown from a saturated solution in DMF (deep-brown solution). The crystals were filtered off, washed with dry i-PrOH and finally dried at room temperature (yield: 47%).
The IR spectrum of the title compound (as KBr pellets) is consistent with the above structural data. It displays the characteristic peak at 1610 cm À1 indicating the formation of a Schiff base (-H-C N-) (Esteves-Souza et al., 2006). The strong bands at 1330-1470 cm À1 can be attributed to overlapped C-H bending (scissoring) (as well in CH 3 groups of the solvent molecule) and aromatic -C C-stretching vibrations. Other strong bands at 1228 and 1244 cm À1 are due to the phenolic CO stretching (Wu et al., 2014). Two medium intensity bands observed at 620 and 738 cm À1 could be assigned to the asymmetric and symmetric C-S stretching vibrations, respectively. In the 1 H-NMR spectrum, the azomethine proton peak that confirms the Schiff base formation is attributed to a singlet signal at 7.9 ppm. It overlaps with the solvent (DMF) proton signal. The O-CH 3 protons peaks only appear at 3.92 ppm. The multiplets of the aromatic protons appear in the range 6.39-6.79 ppm with different multiplicity and coupling constants. The strong singlet at 3.39 ppm could be assigned to the aliphatic -CH 2 -CH 2 -protons according to its integral intensity. Signals from the DMF methyl protons appear at 2.94 and 2.78 ppm. Analysis calculated for for C 33 H 40 N 4 Ni 3 O 7 S 3 (877.00): C, 45.20; H, 4.60; N, 6.39; found: C, 45.5; H, 4.77; N, 6.25.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. All hydrogen atoms were added at calculated positions (C-H = 0.93-0.97 Å ) and refined using a riding model with U iso (H) = 1.2-1.5U eq (C).  The crystal packing of the title compound. C-HÁ Á Á edge-to-face interactions,stacking interactions and the intermolecular C-HÁ Á ÁNi contact that link the components in the crystal are shown as dashed lines.

Antibacterial screening
(EUCAST). The results obtained indicate that the synthesized compound possesses a broad spectrum of activity against the tested microorganisms and shows relatively better activity against Gram-negative than Gram-positive bacteria. The title complex showed activity with lowest minimum inhibitory concentrations (MIC) values 312.5 mg ml À1 against Gramnegative bacteria E. coli, K. pneumoniae and P. aeruginosa. The highest activity was against clinical strain A. baumannii -MIC = 156.2 mg ml À1 . The poorest activity of the complex was against clinical strain Staphylococcus aureus (MRSA). It is well known that A. baumannii is one of the most important nosocomial pathogens because of its longevity in the hospital environment and ability to resist various antimicrobial agents, such as resistance to broad-spectrum -lactam antibiotics by -lactamases production (Peleg et al., 2008;Jamulitrat et al., 2007;Li et al., 2007). The antibacterial study revealed that the title compound has antibacterial activity, the best being against A. baumannii.

Tris(µ 2 -2-methoxy-6-{[(2-sulfidoethyl)imino]methyl}phenolato)trinickel(II) dimethylformamide monosolvate
Crystal data Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.